When a broadcast signal is reproduced, converted from a source format to an output format, or otherwise processed, the timing of the video portion of the signal may deviate from the timing of the audio signal unless care is taken to maintain the audio and video in synchronization. The resultant differential is often referred to as a “lip sync” error, as the viewer of the broadcast signal often detects the timing differential when the broadcast signal contains a representation of a person speaking; the sound of the spoken words is no longer in “sync” with the speaker's lip movement. This problem is experienced not only in consumer devices, but also in commercial broadcast signal processing equipment. Generally, it is desirable to reduce, if not eliminate, lip sync errors because they detract from the viewer experience. Reduction of lip sync error is usually accomplished by delaying the audio signal by a predetermined amount, as the video signal lags behind the audio signal. Under certain processing conditions, the audio may lag the video signal, so the video signal would then have to be delayed.
Previously, methods for reducing the timing error between the audio and video portions of a signal have included manual adjustment based on a delay factor determined by observation by an operator, or automatic adjustment based on a previously determined delay factor. The disadvantage of a manual measurement and adjustment is that it is based on a human-perceived delay; because individuals may have different perceptual thresholds with respect to lip sync error, a manually-determined correction may be not adequate. Prior art methods of automatically delaying the audio by a predetermined factor typically employed an arbitrary delay factor, based on the expected delay in the video signal during processing. This is an inadequate solution because the audio and video signals may be routed through a number of devices or may undergo a number of processing steps that were unknown at the time the arbitrary factor was determined. Each additional device or step may impact the ultimate lip sync error. In particular, when a broadcast signal is processed using a multifunction device, which may provide for multiple signal paths and conversion between a multiplicity of broadcast signal formats, the delay factor may not be predictable. Other prior art methods of detecting a lip sync error included the insertion of a video signal in sync with an audio “pip” (a machine-perceptible signal), and detecting the video and audio signals; however, these prior art methods require specialized equipment and moreover the lip sync detection signals cannot survive the demands of digital broadcast signal processing.
It is therefore desirable to provide a system and method for measuring lip sync error in a signal path that does not require specialized equipment. It is further desirable to provide a lip sync error test signal that is robust enough to survive the rigours of a digital broadcast environment that includes up-, down-, and cross-conversion between formats, analog to digital and digital to analog conversion, as well as compression.